It has been recognized that for a given contact surface area between a liquid and a gas the ability of the gas to dissolve in the liquid increases directly proportionally to the hydrostatic pressure. For this reason it is often desirable in waste water aeration installations to aerate the water below the water surface. It has long been known, for example, to introduce air at superatmospheric pressue through fine-pore distributing elements placed below the liquid surface. This technique requires considerable energy and is often unreliable.
It is also known that aeration can be accomplished by operating a stirrer in a bell under the water surface and feeding as much air to the bell as can be introduced into the water by the stirrer. The stirrer, which may be for example a rotating cylindrical brush or a rotatable surface aerator device agitates the water and the air. Examples of these types of aeration systems are disclosed in published German patent specifications Nos. 1800315 and 2216917. These systems suffer from the disadvantage that the flow of liquid circulating in the basin is not brought about by the aeration means and that excess air escapes from the bell in the form of coarse bubbles.
In co-assigned abandoned United States application Ser. No. 437477, file Jan. 28, 1974 there is described a device for aeration of waste water in a polygonal or rounded basin in which one or more water jets are supplied from above the water surface at a certain angle with the water surface and with the average directon of flow of the waste water. The results obtained in the industrial application of this process have proved effective. The energy cost of this process offers advantages compared with those of processes in which the aeration and suspension of active sludge are combined, but the energy cost still makes up a large portion of the total cost of waste water purification.
It is also known that waste water can be subjected to gas-treatment with the aid of one or more ejectors, in which air is aspirated and distributed in the water by means of a water jet. At the same time, the water in the basin can be kept in motion. The drawback of this is that high water velocities have to applied in order to introduce a sufficient amount of air. Owing to the high water velocity the sludge becomes degraded.